A high speed turbo machine, such as, for example, a steam or gas turbine, generally comprises a plurality of blades arranged in axially oriented rows, the rows of blades being rotated in response to the force of a high pressure fluid flowing axially through the machine. Due to their complex design, natural resonant mechanical frequencies of the blades may coincide with or be excited by certain blade rotational speeds and rotational harmonics thereof. To prevent excessive vibration of the blade about its normal position, prudent design practice dictates that the blades be constructed such that the frequencies of the lowest modes fall between harmonics of the operating frequency of the turbine. In addition, the blades may be excited by non-synchronous forces such as aerodynamic buffeting or flutter. In order to avoid the vibration exceeding certain levels and setting up objectionable stresses in the blades, it is common to monitor the vibrations of the blades, both during the design and testing of the turbine and during normal operation of the turbine. For example, it is known to use non-contacting proximity sensors or probes to detect blade vibrations. The probes detect the actual time-of-arrival of each blade as it passes each probe and provide corresponding signals to a blade vibration monitor system (BVM). Small deviations due to vibration are extracted, from which the BVM may determine the amplitude, frequency, and phase of the vibration of each blade.
In a known blades analysis technique, a system of one or more stationary air jets is commonly employed to provide vibration excitation, i.e., a driving force, to rotating turbine blades mounted on a disk placed within a vacuum spin pit. In this known excitation technique, the air jets excite the turbine blades at a multiple of the disk rotational speed. The disk rotational speed is ramped so as to cover a band of vibrational excitation frequencies. The frequency response of the blades may be detected using a blade tip vibration monitor, such as the BVM described above.
The known blade excitation techniques have several shortcomings. Specifically, only synchronous blade excitation frequencies are produced, i.e., multiples of disk speed. High measurement noise is inherent in synchronous blade tip measurements due to the addition of target structure and sensor placement contributions to the blade pass signals, specifically at synchronous frequencies. Also, multiple sensors are required to measure the synchronous frequencies, contributing to an increased equipment cost. Finally, since the blade frequency response is measured by ramping up the rotational speed of the blades, the blade responses are measured at speeds that are far from the operating speed for the blades. Hence, the blade resonances are measured at reduced centrifugal loading and blade untwist, which may result in an inaccurate characterization of the blade resonances.